As the fiber optic industry has matured, specialty fibers such as erbium-doped fibers, dispersion compensating fibers, fibers containing bragg gratings, and long period grating fibers have become increasingly more important in photonic light-wave systems. To provide the necessary performance, these and other specialty fibers need to be connected (or spliced) to other optical fibers or optical devices without exhibiting excessive connection losses, or "splice losses" as they are known to those skilled in the photonic light-wave system art. Invariably, these specialty fibers have mode field diameters that differ in size and other aspects from the mode field diameters of the fibers or devices to which the specialty fibers will be connected. The connection of fibers having such mismatched mode field diameters generally results in excessive splice loss. Standard single mode fiber, the most commonly used fiber today, is no exception.
A number of techniques have been developed over the years to limit the adverse effect of splice loss resulting from mode field diameter mismatch. Heretofore, physical tapering, in-line optical devices, and thermally diffused expanded core ("TEC") methods have been employed in an attempt to adequately match the mode fields of fibers and other devices having different mode field diameters. Physical tapering includes both down-tapering and up-tapering. In-line optical devices include simple optical devices such as lenses, as well as beam expanding fibers combined with micro optical devices such as isolators and modulators. TEC methods include those methods used to expand the mode field diameter via diffusion.
In the down-tapering method, the optical fiber is first fusion-spliced by conventional methods, and the spliced portion of the fiber is thereafter heated so that it can be stretched by pulling. In this way, the softened spliced part develops a tapered shape. The reduced core misalignment due to the tapered shape and the spreading of the mode field diameter in the smaller core diameter fiber typically result in lower splice loss when compared to the original non-tapered splice. However, the tapers fabricated by this method are sensitive to physical perturbations or external refractive-index change because the mode field is no longer tightly bound to the core. In addition, the outer diameter of the tapered fiber changes during the drawing process, thus special fiber plugs are typically required for any connections.
Unlike the down-taper method, the up-taper is fabricated at the stage of drawing a preform and results in an enlarged core. The enlargement of the core results in the expanded mode field diameter. This method is typically applicable for mechanical splicing, bonded splicing, or connectors between an erbium-doped fiber ("EDF") and an ordinary single-mode ("SM") fiber. However, this method also requires special plugs for the connectors, and in addition necessitates a special preform.
Most in-line optical devices utilize lens elements that collimate a beam from a transmitting fiber, or focus the expanded beam onto the core of the receiving fiber. Others combine devices such as laminated polarizer's, microisolator chips, or modulators embedded within the fiber with thermally induced dopant diffusion in certain specialty fibers. Both of these methods, however, are complicated, unstable, and expensive. In addition, for devices utilizing a lens, alignment is a critical concern.
The thermally diffused expanded core method uses the phenomenon of dopant diffusion in a heated fiber to expand the mode field diameter. The general approach to the fusion connection of two fibers with different mode field diameters is to continuously or adiabaticaly vary the core diameters of one or both fibers so that the mode field diameters match at their boundaries. During the process of dopant diffusion, the core diameter becomes large locally, and the relative refractive index difference becomes small locally compared to the ordinary fiber part. The result is a tapered core and thus tapered mode field diameter within the fiber. Accordingly, the thermally diffused expanded core method can be an effective method for locally expanding the fiber mode field diameter. However, as further discussed below, the thermally diffused expanded core methods heretofore known in the art are not effective for certain applications.
Methods for implementing the TEC technique generally fall into one of two categories. The first is to heat treat the small mode field diameter fiber in a furnace or a gas burner, and then fusion connect the expanded fiber with the larger mode field diameter fiber. The second is to fusion connect the two fibers first, and then apply additional heat to diffuse the fused region. In the first method, furnaces or microburners are generally employed to provide the heat for the diffusion. Due to the temperature limits of most furnaces, the process typically takes several hours to complete, and requires the application of a carbon coating once the primary coating has been stripped from the fiber.
Application of a carbon coating is expensive and time consuming, but is necessary to reduce the heat exposure time required to properly diffuse the dopant. Even though temperatures within the furnace are not generally considered extreme, long periods of exposure to a gas flame tends to make the fiber brittle. For this reason, open-ended furnaces having a maximum temperature of approximately 1300.degree. are employed to treat the fiber. Using such an open-ended furnace generally requires exposing a fiber having a 1% delta for more than ten (10) hours. Because of the low temperature gradient in an open-ended furnace, the fiber core expands slowly along at least 200 mm of the fiber length before reaching maximum diameter. As a result, the long heat-treated section of the fiber has relatively low mechanical strength and requires extra protection and packaging before it can be effectively used in a photonic component. Moreover, because of the large size of the furnace and microburner systems, the first method is not readily available for use in the field where many of the fiber splices must be made.
The second method works well only when the diffusion coefficient of the core dopant in the smaller mode field diameter fiber is much greater than that of the larger mode field diameter fiber. A small mode field diameter fiber doped with erbium is a typical example. For high-delta ("HD") and single-mode fibers, both of which are doped with slowly diffusing germanium, the core discontinuity cannot be completely eliminated using this method. When the splice is fabricated using an arc fusion discharge, the resulting splice loss is typically around 0.3 dB, which is still unacceptably high since there are typically numerous fusion connections of this kind in an optical network. Accordingly, adiabatic coupling cannot be achieved by merely heating the fused region after connection.
In view of the foregoing, there is a need for an optical fiber having an expanded mode field diameter that matches the larger mode field diameter of an optical fiber or other optical waveguide device of a photonic component (or other photonic light-wave systems) so that the fibers can be consistently connected with minimal splice loss. In addition, there exists a need for a method of expanding the mode field diameter of an optical fiber that is easily repeatable, consistent in application, consumes limited time and resources, results in a compact mode field diameter expansion region with respect to the length of the optical fiber, produces minimal splice loss when the expanded mode field diameter fiber is connected to another fiber, and is capable of being performed in the field.